All STD's are bad! Some are curable like syphilis, but some are permanent like herpes, and HIV.

HIV is probably the worst because it will turn into AIDS. AIDS usually isn't what kills people, though - it's an auto-immune disease, so the body attacks itself and because defenceless against even the common cold - and people will die from the cold or the flu.

Don't think of what's the worst, just prevent yourself from getting one. Be in a monogamous relationship, both of you get tested before having sex for the first time, and if you're not a one-person kind of person, then always use a condom. It only takes once, but remember also that a condom isn't 100% at preventing STD's or pregnancy.